Theoretical consideration have shown that ideal photon-counting energy-sensitive detectors applying optimal energy weighting schemes have the potential to increase contrast-to-noise ratio in radiographic images (x-ray radiography and computed tomography) with 20-60% compared to energy integrating systems at the same patient close: Cahn et al, “Detective quantum efficiency dependence on x-ray energy weighting in mammography,” Medical Physics 26:2680-2683 (1999); Shikhaliev, “Projection x-ray imaging with photon energy weighting: experimental evaluation with a proto-type detector,” Physics in Medicine and Biology 54:4971-4992 (2009); and Schmidt, “Optimal ‘image-based’ weighting for energy-resolved CT”, Medical Physics 36(7): 3018-3027 (2009).
The use of spectral information, i.e. assessment about the energy of separate x-ray quanta, also opens up for other spectral medical imaging applications such as quantification of tissue composition: Alvarez and Macovski, “Energy-selective reconstruction in x-ray computerized tomography” Phys. Med. Biol. 21 733-44, (1976)) and k-edge imaging: J-P Schlomka, E Roessl et al. “Experimental feasibility of multi-energy photon-counting K-edge imaging in pre-clinical computed tomography,” Physics in Medicine and Biology 53(15):4031-4047 (2008). The use of spectral information also allow for unproved non-destructive testing outside the realm of medical imaging (security scans) where knowledge of the composition of an unknown material is valuable.
Two main types of direct conversion semi-conductor materials have been proposed for photon counting-mode computer tomography applications: Cadmium Telluride/Cadmium Zink Telluride (CdTe/CZT) and silicon strip detectors (Nowotny, “Application of Si-microstrip-detectors in medicine and structural analysis,” Nuclear instruments and methods in Physics research 226 (1984) pp. 34-39.
The high x-ray flux in a clinical CT examination, up to 1000 Mcps/mm2 on the detector, puts extreme demands on detector read-out electronics and any remaining pile-up will degrade the energy resolution and counting efficiency of the detector and thereby a large fraction of the attainable improvements compared to energy integrating systems. CZT has been shown to suffer from this already at flux rates 100 times lower than those encountered in clinical practice: Barber et al “Characterization of a novel photon counting detector for clinical CT; count rate, energy resolution, and noise performance,” Physics of Medical Imaging, in Proc. of SPIE, vol. 7258 (2009).
The above so called flux rate problem can be simply illustrated: A photon converting in the middle of a 3 mm thick CdTe or CZT detector pixel the induced current pulse will extend 40-45 ns in time. At a count rate of 1000 MHz on the 1 mm2 pixel the average time separation between the pulses will be 1 ns and this explains why pulses will overlap (a phenomenon referred to as signal pile-up) in CdTe detectors already at flux rates well below those encountered in clinical practice.
Silicon as an x-ray detector material has shorter collection times of induced charge carriers (duration of induced currents); for a typical 0.5 mm detector wafer thickness, the collection time is in the order of 8 ns. Silicon is thus less prone to intrinsic pile-up of signals at high flux rates. A smaller pixel size and depth segmentation, particularly with exponentially increasing thicknesses as described in the cross-referenced patent applications U.S. Ser. No. 12/488,930 Jun. 22, 2009 and U.S. 61/151,637 (Provisional) Feb. 11, 2009, further mitigates the problem of signal pile-up.
On the other hand Silicon, when compared to CdTe/CZT, suffers from a relatively low atomic number making it a worse photoelectric absorber. When an x-ray quantum deposits energy by means of the photoelectric effect in a direct conversion detector, to a good approximation all photon energy will be converted to electron hole pairs. In silicon detectors operated at high x-ray energies (>57 keV average photon energy) the Compton effect replaces the photoelectric effect as the dominant type of interaction. For Compton interactions, the deposited energy will depend on the x-ray deflection angle which in turn can only be determined in a statistical fashion using the well-known relationship Klein and Nishina established in 1929. The high fraction of Compton interactions deteriorates the energy resolution; i.e. making it impossible to deduct the original x-ray quantum energy by measuring the deposited energy.
Another benefit of photon counting detector systems is the ability to remove the detrimental effect of electronic noise by applying a low noise rejection threshold. For CdTe/CZT detector systems using typical x-ray spectra with energies ranging from 50 kilo electron volts (keV) to 140 keV, it is highly unlikely that primary quanta will deposit energies below 10 keV. Such a system can therefore apply a relatively high lower threshold around 10-20 keV to reject electronic noise, without the risk of loosing a high fraction of primary x-ray signals.
Due to the above mentioned high fraction of Compton interactions in a silicon detector used at high x-ray energies, many primary x-ray quanta will deposit energy below 20 keV. Applying such a high noise rejection threshold would be severely detrimental to the image quality of such a system since a high fraction of counts from primary x-rays having undergone Compton interaction will be discarded. The noise rejection threshold will therefore need to be set lower.
For very low energy bins, with deposited energy in the order of 0.5-2 keV electronic noise will be the dominant source of false counts in a silicon detector system. Due to Compton scattering in a silicon detector, many primary events depositing low energy will be lost if counts in such low energy bins are simply discarded. For somewhat higher energy bins (with detected energies in the range 2-5 keV), the dominant source of counts pertaining to non-primary events will be signals induced by charges being collected in neighboring pixels or charge shared events. This latter type of noise counts can potentially be removed by application of anti-coincidence logic, but the high flux makes the appropriate coincidence time window short. Including such noise counts high spatial correlation in the image reconstruction will lead to reduced spatial resolution.
The time between primary x-ray quanta reaching the detector after passage through an object will be exponentially distributed with an expected value corresponding to the 1/(flux rate). Pile-up is therefore not a binary phenomenon, i.e. occurring or not occurring. Even if great care is taken to reduce pixel count rate (by means of using smaller pixels and possibly layer the pixel in the direction of the incoming primary x-ray quantum) some pile-up will inevitably occur since there will always be a positive probability for two induced current pulses to overlap. The problem is aggravated by the readout electronics where pulses are integrated over time resulting in signals shapes appreciably longer than the charge collection time in the semi-conductor material itself. This increases the risk of pile-up.
There is thus a need to improve the performance of photon counting imaging systems, with respect to matters such as; loss of energy information for events where deposited energy varies substantially, but in a statistically known fashion, from actual x-ray quantum energy (for instance Compton scattering of k-edge fluorescence in the detector), the detrimental effect on pile-up introduced by the readout electronics and finally the problem of noise counts in the low energy bins.